Lipidic Protic Ionic Liquid CrystalsGuilherme J. Maximo,† Ricardo J. B. N. Santos,‡ Jose A. Lopes-da-Silva,§ Mariana C. Costa,∥

Antonio J. A. Meirelles,† and Joao A. P. Coutinho*,‡

† Laboratory of Extraction, Applied Thermodynamics and Equilibrium, School of Food Engineering, University of Campinas,13083-862, Campinas, Sao Paulo, Brazil‡ CICECO, Chemistry Department, University of Aveiro, 3810-193, Aveiro, Portugal§ QOPNA, Chemistry Department, University of Aveiro, 3810-193, Aveiro, Portugal∥ School of Applied Sciences, University of Campinas, 13484-350, Limeira, Sao Paulo, Brazil

*S Supporting Information

ABSTRACT: Protic ionic liquids (PILs) based on lipidic compounds have a range ofindustrial applications, revealing the potential of oil chemistry as a sustainable basis for thesynthesis of ionic liquids. PILs of fatty acids with ethanolamines are here disclosed to formionic liquid crystals, and their mixtures with the parent fatty acids and ethanolaminesdisplay a lyotropic behavior. Aiming at characterizing their rheologic and phase behavior,four ethanolamine carboxylates and the mixtures used for their synthesis through aBrønsted acid−base reaction are investigated. Their phase diagrams present a complexmultiphase profile, exhibiting lyotropic mesophases as well as solid−liquid biphasicdomains with a congruent melting behavior. These PILs present a high self-assemblingability and a non-Newtonian behavior with yield stress in the liquid crystal mesophase.The appearance of lamellar and hexagonal structures, with probably normal and invertedconfigurations in the mixtures, due to the formation of the PILs is responsible for the highviscoelasticity and notable nonideality that is mainly ruled by hydrophobic/hydrophilicinteractions. Considering their renewable origin, the formation of liquid crystallinestructures, in addition to the non-Newtonian behavior and ionic liquids properties, and the mixtures here evaluated possess agreat potential, and numerous applications may be foreseen.

KEYWORDS: Ionic liquid, Solid−liquid equilibrium, Fatty acid, Ethanolamine, Liquid crystal, Rheology

■ INTRODUCTION

More than a remarkable presence in the open literature duringthe past decade, ionic liquids and mesotherm (i.e., low melting)salts have long since been embodied in a large range ofdetergents, personal care cosmetic, pharmaceuticals, andchemical-marketed products. The interest for ionic liquids,salts with melting temperatures below 100 °C, was driven bytheir peculiar properties: negligible vapor-pressure, wide liquid-phase range, no or limited flammability, and a tunability thatoffers freedom for the design and optimization of chemicalprocesses and product formulation. Such flexibility makes themalso environmentally friendly by reducing the amount ofbyproducts in the industry.1−3 A wide range of mesotherm saltsbased on fatty acids and their derivatives have been used longsince as surfactants, and for many other purposes, withoutactually being recognized as ionic liquids. Protic ionic liquids(PILs) are produced through a Brønsted acid−base reaction(Figure 1) and, therefore, are capable of particular chemicalinteractions, including proton acceptance and proton donation.Because they are easily prepared, often from low-cost or evenrenewable reactants, PILs may be used in several industrialapplications besides their use as surfactants, such as the design

of electrolyte fuel cells, biocatalytic reactions, separation, andself-assembly processes.4−6

Addressing the claims of sustainable chemistry, the search fornatural sources for the synthesis of ionic liquids with lowenvironmental impacts seems to be more than a forthcomingtrend; it seems to be an imperative demand.7−9 In this way,taking into account the renewable aspects and relevance ofseveral industrial sectors in the actual worldwide economicscenario, oil chemistry shows great potential to serve as asustainable supplier of green chemicals. In fact, nowadays, theworld production and consumption of vegetable oils is largerthan 150 millions of tons.10 Considering not only the refiningprocess but also several other steps of the oil production chain,fatty acids emerge as a widely relevant byproduct. Being edible,chemically attractive due to the particular properties of theirlong carbon chain, and having low toxicity, they are widely usedby the industry as the anion for the production of salts, andpotentially of PILs, with properties tailored by choice of anappropriate proton acceptor.11−15 Alkanolamines, such as

Received: September 19, 2013Revised: December 12, 2013

Research Article

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mono-, di-, or tri-ethanolamine, are compounds naturally foundin phospholipid biological membranes. They are often taken asthe cation of the acid−base reaction with fatty acids to producesoaps, which are actually PILs, used in the formulation ofindustrial and hand cleaners, cosmetic creams, aerosol shavefoams, and industrial lubricants due to their excellentemulsifying, thickening, and detergent ability in oil-in-wateremulsions.16−19 In particular, monoethanolamine oleate haslong been embodied in the pharmaceutical industry. Approvedby U.S. Food and Drug Administration (FDA), it acts assclerosant agent when a diluted solution is administratedintravenously in the treatment of leg veins, bleeding esophagealvarices, mucosal varicosities, and other similar surgicalprocedures.20−23 This is quite curious taking into accountthat ILs’ research is looking for effective pharmaceuticalapplications, up to now with limited success.24

These ionic liquids with long alkyl chains and their mixtures,as happens to other surfactant compounds, present thermo-tropic and lyotropic liquid crystalline stable mesophases,25−31

respectively, associated to complex rheologic behaviors, oftencharacterized by non-Newtonian flows and specific viscoelas-ticities32−34 that are characteristics that are so far poorlyexploited. Because of their amphiphilic and asymmetric nature,such PILs can organize themselves in oriented crystallinematrices above the melting temperature, despite presentingliquid flow properties, and are thus able to be used assupporting media for chemical reactions, development of self-assembling drug delivery systems, materials for membraneseparation processes, or electrical conducting media.35,36

This work aims at the characterization of the rheologic andthermotropic behavior of four protic ionic liquid and the solid−liquid equilibrium of the binary mixtures of their precursors:oleic acid + monoethanolamine, oleic acid + diethanolamine,stearic acid + monoethanolamine, and stearic acid + diethanol-amine. These compounds were chosen because of thesustainable character of fatty acids, and despite the use ofethanolamine soaps in commercial products, these systems havenot been previously investigated in the literature. This choice ofsystems allows the characterization of the effects of molecularstructure, alkyl chain length, and presence of unsaturation inthe thermodynamic and rheological behavior of these systemsand the protic ionic liquids that they originate.

■ EXPERIMENTAL SECTIONMaterials. Oleic acid, stearic acid, monoethanolamine, and

diethanolamine were supplied by Sigma-Aldrich (St. Louis, MO)with purities greater than 99%. Samples of approximately 1.0 g of oleic

acid + monoethanolamine, oleic acid + diethanolamine, stearic acid +monoethanolamine, and stearic acid + diethanolamine systems wereprepared gravimetrically on flasks using an analytical balance (MettlerToledo, Inc., Columbus, OH) with a precision of ±2 × 10−4 g andconcentration of component 1 varying from x1 = (0.0 to 1.0) molarfractions for the complete description of the phase diagrams. Themixtures were stirred at 10 K above the mesophases domain of thesamples in an oil bath and under injection of nitrogen to avoidoxidation of the compounds. The uncertainty σx of the compositionsx1, obtained by error propagation from the values of the weighedmasses, was estimated as σx = 5 × 10−4 (molar fraction).

Differential Scanning Calorimetry (DSC). The solid−liquidequilibrium (SLE) data of the mixtures were characterized using aDSC8500 calorimeter (PerkinElmer, Waltham, MA) previouslycalibrated with indium (PerkinElmer, Waltham), naphthalene, andcyclohexane (Merck, Whitehouse Station, NJ) at ambient pressure p =(102.0 ± 0.5) kPa. The samples were submitted to a cooling−heatingcycle at 1 K min−1 based on a methodology established for measuringof the SLE of fatty systems.37,38 The temperatures of the thermaltransitions were then analyzed in the last heating run and taken as thepeak top temperatures. The uncertainty of the temperature was σT =0.40 K, and this value was obtained by the average of the standarddeviation obtained by triplicates of melting temperature values of thepure compounds and mixtures of them.

Light-Polarized Optical Microscopy (POM). The melting profileof the liquid crystalline mesophases of the mixtures were evaluatedusing a BX51 Olympus polarized optical microscope (Olympus Co.,Tokyo, Japan) equipped with a LTS120 Linkam temperature-controlled stage (Linkam Scientific Instruments, Ltd., Tadworth,U.K.) that in this study operated between 243.15 and 393.15 K.Samples (2 mg, approximately) were put in concave slides withcoverslips, cooled to 243.15 K, and allowed to stay at this temperaturefor 30 min. After this, the samples were observed in a 0.1 K min−1

heating run. The uncertainty of the temperature obtained from POMmeasurements was taken as not higher than σPOM = 1.0 K. The valuewas estimated by the mean standard deviation obtained by triplicateevaluation of some of the binary mixtures.

Density, Viscosity, and Critical Micellar Concentration (CMC)Measurements. Density and dynamic viscosity measurements of themixtures were performed between 288.15 and 373.15 K using anautomated SVM 3000 Anton Paar rotational Stabinger viscometer-densimeter (Anton Paar, Graz, Austria) as described elsewhere.39 Therelative uncertainty for the dynamic viscosity was 0.35%, and absoluteuncertainty for density was 5 × 10−4 g cm−3. CMC measurements ofthe protic ionic liquids were performed by conductivity using aMettler-Toledo SevenMulti pHmeter/Conductivimeter (MettlerToledo Inc., Columbus, OH). Aqueous mixtures of approximately10−3 M of each protic ionic liquid were prepared with Millipore waterand kept at 298.15 ± 0.1 K with a thermostatic water bath. Otherconcentrations were obtained by successive dilution. The values of theCMC were taken as the interceptions of two successive linearbehaviors in the conductivity versus concentration plot.

Figure 1. Brønsted acid−base reaction between a carboxylic acid and monoethanolamine or diethanolamine for the formation of a protic ionic liquid.R represents the alkylic moiety of the carboxylic acid.

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Rheological Measurements. The stress−strain flow behavior ofthe protic ionic liquids was determined using a Kinexus-pro stresscontrolled rotational rheometer (Malvern Instruments Ltd., Worces-

tershire, U.K.). The curves were obtained with a shear rate rangingfrom 0 to 100 s−1 at 298.15−388.15 K using a stainless steel cone andplate geometry with a diameter of 40 mm, angle of 4°, and gap set to

Figure 2. Solid−liquid equilibrium phase diagram of the (A) oleic acid + monoethanolamine, (B) oleic acid + diethanolamine, (C) stearic acid +monoethanolamine, and (D) stearic acid + diethanolamine systems by (●) differential scanning calorimetry and (∗) light-polarized opticalmicroscopy presenting solid monoethanolamine (SMEA), solid diethanolamine (SDEA), solid fatty acid (SFA), solid protic ionic liquid (SPIL), liquidphase (L), normal hexagonal (HI), lamellar (Lα), and inverted hexagonal (HII) mesophases. Lines are guides for the eyes. Solid lines wereexperimentally characterized, and dashed lines describe hypothetical phase boundaries based on the Gibbs phase rule.

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50 μm. The temperature was controlled (±0.1 K) by a Peltier systembuilt in the lower fixed flat plate.

■ RESULTS AND DISCUSSION

Phase Behavior. The solid−liquid equilibrium (SLE) phasediagrams of the fatty acid + ethanolamine systems exhibit a

complex multiphasic profile sketched in Figure 2. ExperimentalSLE data for these systems are reported in the SupportingInformation (SI). In all the binary mixtures under study, thereis the formation of a protic ionic liquid, i.e., theethanolammonium salt of a fatty acid, according to the protontransfer reaction sketched in Figure 1, as shown by Alvarez et

Figure 3. Light-polarized optical micrographs showing the mesophases textures: (A) oily texture in the stearic acid + monoethanolamine at xfatty acid =0.2, T = 343.15 K; (B) oily streaks in the oleic acid + diethanolamine at xfatty acid = 0.6, T = 293.15 K; (C) maltese crosses in an isotropic backgroupoleic acid + diethanolamine at xfatty acid = 0.5, T = 357.15 K; (D) oily streaks with inserted maltese crosses oleic acid + diethanolamine at xfatty acid =0.4, T = 340.15 K, (E) mosaic texture in the stearic acid + monoethanolamine at xfatty acid = 0.4, T = 363.15 K; and fan-shaped textures in the (F)oleic acid + diethanolamine at xfatty acid = 0.3, T = 298.15 K; (G) oleic acid + monoethanolamine at xfatty acid = 0.7, T = 293.15 K; and (H) oleic acid +diethanolamine at xfatty acid = 0.7, T = 298.15 K.

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al.12 In fact, at the equimolar composition (x1 = 0.5 molarfraction), the thermogram shows no thermal transitions belowthe melting temperature, unlike what was observed for theother mixtures, as reported in the experimental data tables inthe SI. At this concentration, the system comprises only thepure PIL. Considering the formation of such an intermediatecompound, the phase diagrams should be read taking intoaccount two well-defined regions. The left-hand side of thediagram, or the region at x = 0.0 to 0.5 fatty acid molar fraction,describes the equilibrium between the ethanolamine and theprotic ionic liquid. The right-hand side of the diagram, or theregion at x = 0.5 to 1.0 fatty acid molar fraction, describes theequilibrium between the protic ionic liquid and the fatty acid. Itmeans that the four solid−liquid phase diagrams here reporteddisplay an intermediate compound with a congruent meltingprofile and thus represent, indeed, eight equilibrium profiles,namely, monoethanolammonium oleate + monoethanolamine,oleic acid + monoethanolammonium oleate, diethanolammo-nium oleate + diethanolamine, oleic acid + diethanolammo-nium oleate, monoethanolammonium stearate + monoethanol-amine, stearic acid + monoethanolammonium stearate,diethanolammonium stearate + diethanolamine, and stearicacid + diethanolammonium stearate.These systems present several thermal invariant transitions.

In each phase diagram, two of them are related to the eutecticreaction that bounds the solid phase and the other biphasicdomains. The first eutectic is observed in the ethanolamine-richcomposition region of the diagram and the second in the fattyacid-rich composition region. They are thus related to theminimum point of the liquidus line, i.e., the curve that describesthe behavior of the melting temperature of the system. TheTammann plots of these invariant transitions are reported inthe SI. Their behavior is typically observed for eutectictransitions, with the corresponding enthalpy increasing to theeutectic composition and then decreasing after this transition.40

These plots suggest that the systems present a full immiscibilityin the solid phase because enthalpy is close to zero at purecompound concentration. For stearic acid-based mixtures, theextrapolation of the trend is very close to the pureethanolamine melting enthalpy value meaning that the eutecticpoint in such a region is probably very close to pureethanolamine concentration. The large difference between thepure stearic ionic liquid and pure ethanolamine meltingtemperatures supports such an evaluation.Figure 3 presents the principal textures of the lyotropic and

thermotropic mesophases (in the case of mixtures or pure PILat x1 = 0.5, respectively) above the liquidus line. Five types ofprofiles were characterized, those described as oily textures(Figure 3A), oily streaks (Figure 3B), oily streaks with insertedmaltese crosses (Figure 3C), maltese crosses in an isotropicbackground (Figure 3D), and mosaic textures (Figure 3E),related to the appearance of a lamellar mesophase and thosewith well-defined fan-shaped textures (Figures 3F−H) relatedto the presence of hexagonal mesophases. These structures aretypical of the liquid crystalline state.26−28,41,42 Well-definedlamellar mesophases were observed in all systems at a particularcomposition range. Hexagonal mesophases could be clearlyobserved in the oleic acid-based systems at the extremes of theliquid crystal domain, and this is in agreement with the self-assembly behavior of surfactant molecules. In fact, in aqueoussurfactant solutions, hexagonal ensembles are structured at highwater concentration, while lamellar matrices appear atintermediate concentrations.26−28 Thus, considering that the

solute, in this case, is the ethanolamine or the fatty acid, theappearance of hexagonal mesophases before lamellar ones isexpected. Moreover, when the ionic liquid is in solution withthe ethanolamine, the solvent is hydrophilic. Otherwise, withthe fatty acid, the solvent is hydrophobic. Thus, in theethanolamine-rich region, the hexagonal mesophase observedprobably assumes a normal structure, unlike in the fatty acid-rich region where it probably assumes inverted ones. Normaland inverted hexagonal mesophases are known in the literatureas type I and II, respectively. It means that, in normal type Ihexagonal mesophases, the hydrophilic moiety of the surfactantinteracts with the medium, while the hydrophobic moiety isinside of the ensemble. On the other hand, in inverted type IIhexagonal mesophases, the opposite structure is observed.Moreover, as previously mentioned, hexagonal mesophaseswere not observed in the stearic acid-based mixtures. Thisprobably happens because the linear structure of the fatty acidcarbon chain prefers to self-assemble in layers instead ofcylindrical shapes.43 This will be further discussed.Because liquid crystals were not observed in the pure

ethanolamines and fatty acids, such structures could be relatedto the formation of the protic ionic liquids. In fact, oily texturesand maltese crosses were clearly observed throughout the liquidcrystal phase temperature range at x = 0.5 fatty acid molarfraction, i.e., the pure protic ionic liquid. The same textureswere reported in the literature for similar compounds.44−48

They occur due to the amphiphilic nature of the ethanolam-monium soap and the complex lyotropic behavior of some ofthese mixtures, resulting from the different packaging profilesthat depend on the polarity of the coexistent compound, eitherthe polar ethanolamine in the ethanolamine-rich compositionregion or the nonpolar fatty acid in the fatty acid-richcomposition region.Taking into account the existence of these mesophases, it is

possible to characterize the other thermal invariant transitionsthat are observed in the biphasic domain. Oleic acid-basedmixtures (Figure 2A and B) present, at the left-hand side of thediagram, two invariant transitions. The first, at a highertemperature, is related to the formation of the hexagonalmesophase, and the second is related to the formation of thelamellar mesophase. In the right-hand side of the diagrams, theionic liquid-rich region shows one invariant transition related tothe lamellar mesophase formation, and the fatty acid-rich regionpresents one transition, probably related to the formation of theinverted hexagonal mesophase. On the other hand, becausestearic acid-based mixtures (Figure 2C and D) showed onlylamellar mesophases, the invariant thermal transitions in thebiphasic region are related to the formation of such structure.The profile of the phase diagrams, including the solid−liquid

equilibrium and the mesophases, could be sketched in Figure2A−D, considering the experimental observations and theGibbs phase rule imposing that at a constant pressure andtemperature at most three phases can coexist in a binarymixture.40,49−51 Taking into account the coexistent phases andmesophases, namely, solid ethanolamine (SEA), solid fatty acid(SFA), solid protic ionic liquid (SPIL), isotropic liquid (L),normal hexagonal (HI), lamellar (Lα), and inverted hexagonal(HII), it is possible to summarize all biphasic domains that arepresent in the phase diagrams. In the oleic acid +monoethanolamine (Figure 2A) and oleic acid + diethanol-amine (Figure 2B) systems, we have SEA + SPIL, SEA + Lα, SEA +HI, SEA + L, SPIL + SFA, SPIL + Lα, SPIL + HII, SFA + HII, and SFA +L. In the stearic acid + monoethanolamine system (Figure 2C),

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we have SEA + SPIL, SEA + Lα, SEA + L, SPIL + SFA, SPIL + Lα, SPIL+ L, and SFA + L. In the stearic acid + diethanolamine system(Figure 2D), we have SEA + SPIL, SEA + L, SPIL + L, SPIL + SFA,SPIL + Lα, SFA + Lα, and SFA + L.Self-Assembling of PILs. It was mentioned before that the

appearance of the liquid crystal mesophases is due to theformation of the protic ionic liquid. In fact, it presents anamphiphilic and asymmetric molecular structure, containing ahydrophilic (ethanolammonium cation) and hydrophobicmoiety (fatty acid anion). Such profile characterizes a mesogen,i.e., a compound that under suitable temperature and pressureconditions can build liquid crystal matrices.36,42 For theunderstanding of the self-organizing behavior of the proticionic liquids formed by the reaction of the fatty acids withethanolamines mixtures, the critical micellar concentration ofthese compounds in water was measured by conductivity.

Figure 4 shows the conductivities of the diluted aqueoussolutions of the four protic ionic liquids here studied. As theconductivity increases with the concentration of the ionicliquids in solution, the slope changes defining essentially threedifferent linear regions. Thus, considering that the differentdependence of conductivity upon PIL concentration is due todifferences in the self-assembly of the mesogens, two CMCvalues were obtained (as shown in Figure 4). Literature showsthat with an increase in the surfactant concentration in aqueoussolutions, spherical micelles, cylindrical micelles, or bilayers areformed from the monomers.52 After the first CMC value, theprotic ionic liquid is probably self-assembled in sphericalmicelles. In fact, taking into account the molecular geometry(local) constraints, the critical packing parameter53 in all casesassumed values lower than 1/3. This value is related to theformation of spherical micelles with the polar headgroup(ethanolammonium cation) interacting with the aqueousmedium (as depicted in Figure 4). Moreover, considering theglobal constraints imposed by an increase in the concentrationof the mixture and intermolecular interactions, the secondCMC value could be probably related to the formation of morecomplex structures, according to similar observations in theliterature.54−56 The formation of such structures is moreevident in the oleate ionic liquids because they present a morepronounced shift in the slope and lower CMC2 values.Assuming, in this case, both global and local constraints, thepresence of the unsaturation in the carbon chain of the oleic

ionic liquids might probably favor the formation of cylindricalself-assembling. On the other hand, the geometry of thesaturated alkyl chain of the stearic ionic liquids probably led tothe formation of bilayer structures. This observation corrobo-rates with the fact that stearic acid based mixtures did notpresent hexagonal crystals, unlike oleic acid ones, as previouslydiscussed.

Rheological Characterization of PILs and Their BinaryMixtures. Taking into account the organized nature of a liquidcrystal microstructure, its properties should present a particularbehavior, or even a different profile, when compared with anisotropic media. In fact, visually, some mixtures close to x = 0.5fatty acid molar fraction, including the equimolar concentration,presented a very viscous gel-like behavior. The phase diagramsof the mixtures here studied show that above the liquidus line,there is a very large liquid crystalline domain in a broadconcentration range and with maximum temperature valuesclose to 390 K. Thus, density and viscosity measurements weredetermined in order to characterize these fluids, fostering theunderstanding of the interactions between the compounds inthe mixture because in this work the compounds react to forma new substance. Unfortunately, due to their high meltingpoints, the mixtures of stearic acid could not be studied. Figure5 presents the densities of the oleic acid + monoethanolamineand oleic acid + diethanolamine binary systems from 288.15 to353.15 K because such mixtures are liquid above 288.15 K. As afirst observation, the results seem to be quite trivial with densityvalues at fixed concentrations decreasing linearly (R2 ≥ 0.99)with increasing temperature, ranging from ρ = 1.10 to 0.85 gcm−3, i.e., from pure ethanolamine to pure oleic acid densityvalues. However, if the density results are seen on theircomposition dependency at a fixed temperature, the values alsodecrease with an increase in oleic acid concentration, but amore complex behavior is revealed. Three different regions arenow apparent: (1) one in the ethanolamine rich region beforethe liquid crystal formation, (2) one delimited by the liquidcrystal domain (dashed lines of the Figure 5), and (3) one inthe fatty acid-rich region where no liquid crystal is observed.These differences in the density result from the peculiarpacking profiles that liquid crystals present, which inducechanges in the densities of the mixtures. The excess molarvolume VEX/cm3 mol−1 data calculated from the experimentaldensity data (Figure 6) are in agreement with the observednonlinear density dependency and provide further significantdetails on the molecular interactions. It is important to notethat the excess molar volume was calculated considering that atx = 0.0 to 0.5 oleic acid molar fraction the mixture is theethanolamine + ionic liquid binary system and at x = 0.5 to 1.0oleic acid molar fraction the system is the fatty acid + ionicliquid mixture. The results clearly show that, at a fixedtemperature, the excess molar volumes of the ethanolamine +protic ionic liquid mixtures present a significant negativedeviation from ideal behavior. On the other hand, the excessmolar volumes of the oleic acid + protic ionic liquid mixturespresent significant positive deviations. Because negativedeviations were observed in the first part of the diagram, onecan infer that from the point of view of excess molar volumesthe interactions between ethanolamine and the ionic liquid aremore favorable than the interactions between fatty acid and theionic liquid. In fact, ethanolamines and fatty acid have quitedifferent size, shape, and polarities. In the ethanolamine + ionicliquid mixture, due to the polar behavior of the ethanolamine,the interactions with the hydrophilic moiety of the ionic liquid

Figure 4. Conductivity (κ) as a function of concentration of proticionic liquids (PIL) aqueous solutions. Solid lines are linear fitting ofthe data (R2 > 0.98) for critical micellar concentration determination(CMC).

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are preferred. The large size differences may also favor thepacking of the ensemble. On the opposite, in the fatty acid +ionic liquid systems, the interactions with the hydrophobicmoiety of the ionic liquid are preferred due to the apolarbehavior of the organic acid. Thus, the rigid structure of theliquid crystal formed and the large size of the long carbon chainprobably do not allow much possibilities of packing leading topositive deviations from ideal behavior.Figure 7 shows the dynamic viscosity measurements for the

same systems from 288.15 to 358.15 K except at and close tothe equimolar composition for the oleic acid + monoethanol-amine system due to its high viscosity with a gel-like behavior.In the case of pure PILs, shear rate-dependent rheologicalmeasurements were conducted and will be later discussed. Anexponential decay of the viscosity with temperature wasobserved, being more pronounced close to the pure ionic

liquid. This is clearly visible considering the isothermal behaviorat 353.15 K of the viscosity with the fatty acid concentration atthe bottom of Figure 7. The viscosity values increased with theconcentration from pure ethanolamine composition to theprotic ionic liquid, and then they start to decrease again to thevalue of pure oleic acid. Another singular behavior from theviscosity measurements can be highlighted if the data is fitted toan Arrhenius-like exponential equation (eq 1)

η = +AERT

ln( ) ln( ) A(1)

where η is the dynamic viscosity (Pa s), EA is the activationenergy (J mol−1), R is the gas constant (8.314 J mol−1 K−1), Tis temperature (K), and A is the pre-exponential factor (Pa s).Thence, Figure 8 shows the logarithm of the viscosity data, ln(η), as a function of T−1. The fitting procedure, which ispresented at the right side of Figure 8, shows that the trendpresents a shift with a significant change in the activationenergy EA at a temperature (Tinters in Figure 8) close to themelting of the mesophases. Considering the meaning of thecoefficients of the Arrhenius equation, the results reveal that theactivation energy of the liquid crystalline state is higher thanthat in the isotropic liquid phase. The crystalline mesophaseneeds, thus, more energy to flow than the isotropic liquid.57

Figure 9 shows some of the flow curves of the oleate-basedPILs (x = 0.5 oleic acid molar fraction) at 293.15 to 363.15 K,describing the shear rate-dependence of the samples’ shearstresses. For the monoethanolammonium oleate sample at T >348.15 K, the shear rate is directly proportional to the shearstress, with no evidence of yield stress (σ0), characterizing aclear Newtonian behavior (eq 2). The same result is observedfor diethanolammonium oleate samples at T > 323.15 K. Belowsuch temperature values, for both PILs, the increase in shear

Figure 5. Density data (ρ/g cm−3) as a function of temperature (T/K) for the (A) oleic acid + monoethalonamine and (B) oleic acid +diethanolamine systems at xoleic acid = 0.0 (×), 0.1 (●), 0.2 (∗), 0.3 (◆), 0.4 (+), 0.5 (▲), 0.6 (○), 0.7 (■), 0.8 (△), 0.9 (□), and 1.0 (◇). At thebottom of each plot are density data as a function of oleic acid molar fraction at 298.15, 323.15, and 353.15 K. Solid and dashed lines are guides forthe eyes. Gray regions represent the liquid crystal domain.

Figure 6. Experimental excess molar volume data of the (○) oleic acid+ monoethalonamine and (×) oleic acid + diethanolamine systems at298.15 K. Dashed lines are guides for the eyes. Vertical gray line at x =0.5 (pure PIL) depicts the “true” system indicated at the top of theplot.

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rates leads to a nonlinear increase in shear stress. At 298.15 ≤ T≤ 348.15 K, the monoethanolammonium oleate samplepresented a significant yield stress (outlined by the dashedcircles in Figure 9), whose values increase with an decrease intemperature from σ0 = 10 to 70 Pa. At this range, the shearrate/shear stress dependence changes gradually to a markednonlinear behavior that could be adjusted by the Vocadlo

model58,59 (eq 3) up to T > 293.15 K. At this low temperature,the yield stress increases to σ0 > 200 Pa with a lineardependence between shear rate and shear stress such that theflow curve is well-characterized by the Bingham plastic model(eq 4, R2 > 0.99). Comparatively, the non-Newtoniantemperature domain of the diethanolammonium oleate samplesdid not present yield stress up to 308.15 K, where the flowcurve is well adjusted (R2 > 0.99) by a shear-thinning profile(eq 5). However, with the temperature decreasing (T < 308.15K), the flow curve presents a yield stress, and the Herschel-Bulkley model (eq 6) could be accurately adjusted (R2 > 0.95).The model’s parameters fitted to flow experimental data are inthe SI. The flow models adopted here60 can be described asfollows

σ ηγ= (2)

σ σ γ= + K( )n n0V

1/V

v V (3)

σ σ γ= + η′ 0 (4)

σ γ= K n(5)

σ σ γ= + K n0H H

H (6)

where σ is the shear stress (Pa), η is the viscosity (Pa s), γ is theshear rate (s−1), σ0, σ0V, and σ0H are the yield stresses (Pa)related to each model, K, KV and KH are the consistency indexesrelated to each model, η′ is the Bingham plastic viscosity, n, nV,and nH are flow behavior indexes related to each model, and thesubscripts V and H make reference to the Vocadlo andHerschel−Bulkley models, respectively. The non-Newtonianbehavior displayed by these PILs is thus a unique characteristicof these compounds, and the flow phenomena here observedplay a key role in numerous industry applications.32−36,57 Theself-assembly ability of the PILs and formation of thermotropicmesophases with bilayered lamellar-oriented structures abovethe melting point are the main reason for the non-Newtonianrheological behavior here described. Moreover, the chemicalinteractions among the molecules appear to be strong enoughfor the requirement of a critical shear stress. The increasingtemperatures lead to a decrease in the apparent viscosity. This,

Figure 7. Dynamic viscosity data of (A) oleic acid + monoethanolamine and (B) oleic acid + diethanolamine at xoleic acid = 0.0 (×); 0.1 (●); 0.2 (∗);0.3 (◆); 0.4 (+); 0.5 (▲); 0.6 (○); 0.7 (■); 0.8 (△); 0.9 (□); 1.0 (◇). Details at the top right of the plots show magnifications of the data at low ηvalues. At the bottom of each plot are viscosity data as a function of fatty acid molar fraction at 353.15 K. Vertical gray lines at x = 0.5 (pure PIL)depict the “true” systems indicated above.

Figure 8. Linearization procedure of the dynamic viscosity η datausing the Arrhenius-like eq 1. (A) Oleic acid + monoethanolanmineand (B) oleic acid + diethanolamine at xoleic acid = 0.0 (×); 0.1 (●); 0.2(∗); 0.3 (◆); 0.4 (+); 0.5 (▲); 0.6 (○); 0.7 (■); 0.8 (△); 0.9 (□);1.0 (◇). At the right of each plot are linear fittings of the data (R2 >0.99) at some concentrations. Gray regions represent the liquid crystaldomain.

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for practical purposes, says there is an attenuation of the non-Newtonian behavior followed by the appearance of aNewtonian profile at sufficiently high temperatures. Thetemperature increase induces the breaking of the interactionsthat support the liquid crystalline microstructure. Theconcentration of the isotropic phase increases with depletionof the mesophase, as observed in the phase diagrams (Figure 1)and in the polarized optical micrographs (Figure 2A, typicallamellar texture at low temperatures; Figure 2C, textures at hightemperatures). It was also observed that the non-Newtonianbehavior of monoethanolammonium oleate is more evidentthan for the diethanolammonium oleate in special at lowtemperatures where the elastic characteristic of such PILfollows a Bingham plastic profile, presenting very high yieldstresses. So, regarding the size of the cation in the surfactantmolecule structure, one might suppose that the bilayersconstructed by the monoethanolammonium oleate are morepacked with stronger intermolecular hydrogen bonding net-works than the diethanolammonium oleate structure, whoselarger cation decreases the self-assembly ability. In fact, complexstructures are more easily formed by monoethanolammoniumoleate due to its lower CMC2 value. The non-Newtonianbehavior exhibited by the ethanolammonium oleates is quiteinteresting concerning their use in the formulation of productsand considering the particular physicochemical properties ofthe protic ionic liquids for specific applications.4−6,12,15−17,35,36

■ CONCLUSIONSFour PILs based on oleic and stearic acids, with mono- and di-ethanolamines as cations, and the binary mixtures used for theirpreparation were here investigated. The protic ionic liquids areshown to display an interesting thermotropic behavior with theformation of lamellar mesophases. The SLE phase diagrams ofthe mixtures of their precursors display a congruent meltingbehavior with lamellar and hexagonal lyotropic mesophases.Moreover, the oleic acid-based systems also present a non-Newtonian behavior clearly observed in the flow curvesobtained by the stress-controlled rheological assays. Althoughmany ethanolammonium carboxylates have a range of industrialapplications known, considering their renewable origin, thenovel characteristics here reported, such as the formation ofliquid crystalline structures in addition to the non-Newtonianbehavior and the unique properties resulting from their ionic

liquid character, confer to the compounds here investigated agreat potential. On the basis of these results, numerousapplications may be foreseen for these compounds, besidesthose currently in use, which should be the object of futurestudies.

■ ASSOCIATED CONTENT

*S Supporting InformationTables with experimental solid−liquid equilibrium data,Tammann plots of eutectic transitions, and rheological models’parameters fitted to experimental flow curves. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Fax: + 351 234 401 470. Tel: + 351 234 401 507. E-mail:jcoutinho@ua.pt.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors are grateful to the national funding agenciesFAPESP (2012/05027-1, 2008/56258-8), CNPq (304495/2010-7, 483340/2012-0, 140718/2010-9, 479533/2013-0), andCAPES (BEX-13716/12-3) from Brazil, and FCT (Pest-C/CTM/LA0011/2013) from Portugal for the financial support.

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Figure 9. Shear stress (σ) versus shear rate (γ) curves of (A) monoethanolammonium oleate and (B) diethanolammonium oleate at 293.15 (○),298.15 (×), 308.15 (▲), 323.15 (■), 328.15 (◇), 348.15 (□), 353.15 (+), and 363.15 (∗) K. Solid lines are fitted curves. Labels N, ST, V, BP, andHB stand for Newtonian, shear-thinning, Vocadlo, Bingham plastic, and Herschel−Bulkley models, respectively.

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